Newer generations of aircraft and other air and space vehicles, including military aircraft, are increasingly using fiber optic connections, rather than metallic wiring, for communications, weaponry, and other systems. Optical fiber connections provide faster communication and increased bandwidth. However, optical fiber can be more fragile than metallic wiring and can be damaged during installation or during the rigors of flight testing.
Damage to optical fiber can be difficult to distinguish from faults occurring in other components of the communication or weaponry systems. Even when it is known that the damage exists, it can be difficult to identify the precise location of the damage. These difficulties can increase the maintenance costs and reduce the operating efficiency of the aircraft.
One solution to decrease the maintenance costs of any complex system that uses fiber optic connections is to implement built-in test (BIT) capability. BIT capability may be incorporated directly into the fiber optic transceiver module that is otherwise responsible for transmitting and receiving signals via the associated optical fiber. In this regard, the fiber optic transceiver module may include hardware and/or software to identify the existence and location of damage to the associated optical fibers, thereby providing the BIT capability. The optical fiber can therefore be routinely tested for any discontinuities in the optical fiber that are indicative of damage. For example, the optical fiber can be tested every time the system is started. This ability to routinely test the optical fiber allows for fast identification and repair of damage, thereby increasing operating efficiency.
Hardware installed in military aircraft must meet exacting military specification (mil spec) requirements. One of the mil spec requirements for fiber optic transceiver modules is that the modules must have a low physical profile so as to occupy minimal space in the aircraft. For example, one current mil spec requirement limits the height of fiber optic transceiver modules to 140 mils or less (one mil equals one thousandth of an inch). This is in compliance with the requirements of the Standard Electronic Module Type E format for circuit cards used in line replaceable modules in commercial aircraft and in weapon replaceable modules in military aircraft.
One known method of implementing a low profile fiber optic transceiver is to use an optical fiber having an angled end face. The end face may be disposed at a variety of angles, but is typically disposed at about an angle of 45° relative to the longitudinal axis of the optical fiber. An optical fiber having an angled end face can be used to couple optical signals between various devices, such as various optoelectronic devices, including, for example, a photodiode or a vertical cavity surface emitting laser (VCSEL). The VCSEL controlled by a transmitter application specific integrated circuit (ASIC) would emit an optical signal in a first direction. The optical fiber would be positioned in an aligned relationship above the VCSEL so as to receive the optical signal and to transmit the optical signal to another device, such as another optoelectronic device including, for example a photodiode that functions as a receiver. Thus, this optical fiber establishes the communication link between the VCSEL-based transmitter and the photodiode receiver.
The optical fiber would receive the optical signal emitted by the VCSEL via an angled end face which redirects the optical signal along the length of the optical fiber, as described below. The angled end face of the optical fiber permits the optical signal emitted by the VCSEL to be totally internally reflected within the optical fiber as indicated by the dashed line. The fiber optic transceiver module would be contained in a housing and would exit the housing through a hermetically-sealed feed-through.
Advantageously, the spacing between the end face of the optical fiber and the VCSEL may be quite small, such as 50-100 microns, thereby potentially reducing the size and profile of the resulting package. Moreover, the optical fiber may be oriented such that the longitudinal axis of the optical fiber extends perpendicularly to the direction of the signals emitted by the VCSEL, thereby reducing the profile or height of the fiber optic transceiver module. The optical signal emitted by the VCSEL propagates through the side surface of the optical fiber and is internally reflected by the angled end face so as to then propagate lengthwise along the optical fiber. Thus, the angle at which the end face is disposed relative to the longitudinal axis of the optical fiber may vary, but is generally defined by Snell's law, such that the optical signal is totally internally reflected within the optical fiber.
By using an optical fiber having an angled end face, the optoelectronic devices, such as a VCSEL, a photodiode, or other optoelectronic device, may be disposed in a relatively small package having a low profile. The fiber optic transceiver module may also include other components, such as optoelectronic devices and other devices, including ASICs. The spacing between the VCSEL and the other components is also advantageously minimized, thereby allowing shorter electrical bond wires. Shorter bond wires correspondingly reduce parasitic capacitance and inductance, thereby increasing the potential speed at which the system may operate. The known fiber optic transceiver module described above achieves the required low physical profile, but does not incorporate BIT functionality.
In the absence of an optical fiber having an angled end face, the optical signal emitted by the VCSEL would generally be coupled to an optical fiber having a planar end face that extends perpendicularly to the longitudinal axis of the optical fiber itself. One known configuration would position the VCSEL with the fiber extending upwardly from the VCSEL for some distance before being gradually turned and redirected in the desired direction using bulk optical elements. Another known configuration would use a VCSEL mounted on a header such that the optical signal emitted by the VCSEL is oriented along the same axis as the optical fiber. Either of these known configurations would significantly increase the profile or height of the fiber optic transceiver module. Additionally, BIT functionality could not be implemented with either of these configurations.
While the conventional fiber optic transceiver described above provides a low physical profile to meet mil spec requirements, it does not provide BIT capability. Therefore it would be desirable to have a fiber optic transceiver module that meets the mil spec requirement of a low physical profile and also has built-in-test capability.